CoAuthors

Ganguly D. (SAC, ISRO, India); Desai Shruti (SAC, ISRO, India); Chauhan Prakash (SAC, ISRO, India)

Altimeters were designed to work over Open Ocean. Over inland water bodies due to large footprint and land contamination, its accuracy is limited to few sub meters to meters. But with the advancement of technology and introduction of new frequency, now it is possible to measure water level fluctuation within cm level accuracy. Our study area is Ukai Dam/Reservoir, 612 km2, on an average 6km wide. SARAL altimeter passes 6km away from the Dam boundary and along track distance is nearly 10km. A Landsat-8 image of Ukai reservoir along with the first 9 passes of SARAL is shown in figure. Firstly we have demonstrated the capability of previous altimeters over the study area. Envisat data over the reservoir was processed for all the available retrackers and results were validated with the in-situ dataset provided by Dam authority. LEGOS produced Hydrology web product over the reservoir was also compared with the results. Overall ICE-1 retracker works best with minimum RMSE 1.06 m, while LEGOS hydrology product RMSE was found to be 3.06 m.
In this study we have proposed new methodology to measure water level over inland bodies based on averaged high rate waveforms, new retrackor and modified range corrections. These results were then validated with the GPS survey and in-situ tide gauge dataset. SARAL derived water level information from all the retracker available in the GDR product were compared with the in-situ tide gauge dataset installed close to the Dam. Averaged high rate waveforms were analyzed for better performance, i.e. single 40Hz, 10 40-Hz and 1 Hz combinations. ECMWF reanalysis pressure fields and Global ionosphere maps were used to exactly estimate the range corrections. Overall ICE-1 retracker works best with RMSE within the range of 32 cm. A field trip was conducted on the 17th January 2014, same day on the SARAL pass, using two Dual frequency GPS instruments. One GPS was mounted close to Dam as static mode and the other was used on a moving vehicle within the reservoir in Kinematic mode. Crossover points of GPS and altimeter pass were compared and validated with tide gauge measurement. A new retracker, especially for the multi-peak echo that is the most common waveform encountered over lakes and reservoirs, has been proposed based on the assumption that out of all the waveform peaks one of them corresponding to actual nadir surface. New retrackor work best with overall RMSE within the range of 8.8 cm. The results supports that AltiKa dataset can be utilized for more accurate water level information over inland water bodies.
Keywords: Waveform retracking, ICE-1(Offset Centre of Gravity retracker), GPS Kinematic mode